Your argument demands an obviously immoral crime in order to work. You used shoplifting because everyone agrees that is immoral. You are begging the question.
If you replaced shoplifting with cannabis possession then the line "If everyone started smoking pot, for example, it wouldn't indicate a problem with the laws that people are now criminals" isn't nearly true. In fact, you could argue that alcohol is legal, while cannabis is not, is entirely because everyone does it.
1. I don't agree with the statement "shoplifting is necessarily immoral" so it makes no sense for my own argument to hinge on such a fact.
2. My argument does not only work for an immoral crime; it is merely illustrated by an example of a law people think is not wrong. Shoplifting is merely a counterexample to the logic "If X is a law that a lot of people break then X is a bad law". I do not take this on to conclude that X is not a bad law which would be begging the question.
3. The original logic was begging the question, not I. The statement "If X is a law that a lot of people break then X is a bad law" only holds if X is already a priori a bad law (and, even then, it requires a few more statements to actually work).
> Shoplifting is merely a counterexample to the logic "If X is a law that a lot of people break then X is a bad law"
Perhaps if 90% of the population shoplifted, then any law against shoplifting would be bad. What is missing from the question is why the majority of the population would shoplift.
That is a very valid line of reasoning and one that I would implore everyone to explore in such a situation. But, as you said, "perhaps". It does not directly follow that it is necessarily a bad law.
If you replaced shoplifting with cannabis possession then the line "If everyone started smoking pot, for example, it wouldn't indicate a problem with the laws that people are now criminals" isn't nearly true. In fact, you could argue that alcohol is legal, while cannabis is not, is entirely because everyone does it.